Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 171722
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1021 AM MST WED DEC 17 2014

.SHORT TERM...WEDNESDAY THROUGH FRIDAY NIGHT

THE TWO CONCERNS OF YESTERDAY ARE PRETTY MUCH THE MAIN CONCERNS OF
TODAY. ONE IS FOR THE DREADED FOG MONSTER. THE WORST AREA IS
ACTUALLY RIGHT AROUND THE HUMBLE RIVERTON WEATHER ABODE WHERE
VISIBILITY HAS BEEN DOWN AROUND A QUARTER OF A MILE FOR SEVERAL
HOURS. WE HAVE A DENSE FOG ADVISORY OUT THROUGH 9AM AND WE WILL
CONTINUE IT FOR NOW. WITH THE BUFFALO AIRPORT SEEING VISIBILITY
DOWN TO A QUARTER OF A MILE...WE WILL EXPAND THE DENSE FOG ADVISORY
THERE AS WELL. ELSEWHERE...WE HAVE PATCHY FOG IN ACROSS THE
FAVORED AREAS AND WE WILL CONTINUE THAT IN THE FORECAST AS WELL.
THE NEXT QUESTION IS WILL THE LOWER CLOUDS MIX OUT AS A SHORTWAVE
AND SOME WEAK COLD ADVECTION MOVES IN. FOR NOW...WE LEFT THINGS
MOST CLOUDY IN THE MORNING WITH SOME SLOW CLEARING IN THE
AFTERNOON. WITH CLOUDS AROUND WE ALSO KEPT THE TEMPERATURES BELOW
MOS GUIDANCE. WITH LIGHT WINDS EXPECTED...WE ALSO KEPT PORTIONS OF
THE BIG HORN BASIN BELOW GUIDANCE WITH THE INVERSION NOT EXPECTED
TO BREAK.

MEANWHILE...SOME SNOW IS CONTINUING EARLY THIS MORNING AS THE
AFOREMENTIONED SHORTWAVE MOVES THROUGH THE ARE. RADAR TRENDS HAVE
BEEN DECREASING THROUGH THE MORNING AND WITH SOME SHORTWAVE RIDGING
EXPECTED TO BUILD INTO THE AREA POPS SHOULD DECREASE THROUGH THE DAY.

AS FOR THURSDAY...ANOTHER WEAK SHORTWAVE WILL MOVE ACROSS WESTERN
WYOMING. THE MODELS DO HAVE SOME DIFFERENCES WITH THE GFS A BUT
STRONGER...FASTER AND WETTER THAN THE NAM. FOR NOW...WE BLENDED BOTH
MODELS. SOME MORE SNOW SHOWERS LOOK LIKE A DECENT BET...ESPECIALLY
LATE IN THE DAY AND AT NIGHT BUT AGAIN...AMOUNTS DO NOT LOOK VERY
IMPRESSIVE AT THIS TIME. MEANWHILE...EXPECT A DRY DAY EAST OF THE
DIVIDE WITH A BIT MORE SUNSHINE. AGAIN...WITH THE WEAK SUNSHINE AS
WE APPROACH THE WINTER SOLSTICE...WE KEPT HIGH TEMPERATURE BELOW MOS
NUMBERS ACROSS THE BASINS WITH INVERSIONS EXPECTED TO BE TOUGH TO
BREAK WITH LITTLE MIXING. BY FRIDAY AND ESPECIALLY FRIDAY
NIGHT...THE NEXT IN A SERIES OF SHORTWAVES WILL MOVE INTO WESTERN
WYOMING. THERE ARE SOME DIFFERENCES IN THE DETAILS WITH THE NAM ONCE
AGAIN MORE BULLISH WITH QPF. AGAIN...WE BLENDED THE MODELS AT THIS
POINT ALTHOUGH AMOUNTS LOOK TO REMAIN SUB ADVISORY AT THIS POINT.
THE GFS HAS AN INTERESTING WRINKLE IN BRINGING SOME SNOW EAST OF THE
DIVIDE FRIDAY NIGHT IN ASSOCIATION WITH SOME JET ENERGY. SOMETHING
INTERESTING THAT NEEDS TO BE WATCHED.


.LONG TERM...SATURDAY THROUGH WEDNESDAY

ACTIVE MEDIUM RANGE PERIOD WITH A COUPLE SIGNIFICANT STORM SYSTEMS
TO WATCH CLOSELY IN THE EARLY TO MIDDLE PART OF NEXT WEEK. ON
SATURDAY...A FAIRLY WEAK SHORTWAVE IS FORECAST TO MOVE EAST ACROSS
THE AREA WITH A LITTLE MOUNTAIN SNOW AND PERHAPS A FEW FLURRIES OR
SPRINKLES IN THE FAR NRN ZONES. THINGS GET MUCH MORE INTERESTING
SUNDAY THROUGH MONDAY AS VERY STRONG UPPER LOW AND 190KT JET MOVE
FROM THE WRN PACIFIC INTO THE WESTERN U.S. BY THE END OF THIS
PERIOD. MOIST ISENTROPIC UPGLIDE PATTERN DEVELOPS LATE SATURDAY
NIGHT OR SUNDAY MORNING IN THE NW AND EXPANDS TO MOST OF THE WEST
SUNDAY WHILE JET COMES ON SHORE WITH AT LEAST 140-150KT SPEEDS.
THREAT OF A SIGNIFICANT SNOWSTORM WITH INCREASING WIND IS STILL
EXPECTED FOR THE WESTERN MTN SUNDAY INTO MONDAY. TEMPERATURES WARM
CONSIDERABLY SO SOME OF THE WRN VALLEYS...ESPECIALLY THE STAR VALLEY
MAY SEE A RAIN/SNOW MIXTURE BY SUNDAY AFTERNOON AND NIGHT. INCREASED
POPS TO LIKELY OR HIGHER IN MOST OF THE FAR WEST FOR THIS STORM
SYSTEM. AT THE SAME TIME...WIND WILL BE INCREASING EAST OF THE
DIVIDE WITH LEE TROUGHING DEVELOPING IN CONJUNCTION WITH 50-70KT MTN
TOP WINDS AND 140-150KT H3 JET. POTENTIAL HIGH WINDS IN THE WIND
CORRIDOR...ABSAROKAS AND CODY FOOTHILLS SUNDAY INTO MONDAY MORNING
SHIFTING TO THE NW FAVORED AREAS LIKE DUBOIS TO RIVERTON LATER
MONDAY INTO MONDAY NIGHT. POTENTIAL RAIN AND OR SNOW WITH THIS
SYSTEM...ESPECIALLY ON THE NORTH SIDE OF THE JET ON MONDAY. EVEN
SUNDAY AS STRONG UPGLIDE IMPACTS THE NORTH...WE COULD SOME RAIN OR
SNOW IN THE NORTH.

AFTER THIS SYSTEM PASSES BY...ALL EYES TURN TOWARDS POTENTIAL
SNOWSTORM FOR CHRISTMAS EVE INTO CHRISTMAS DAY. THIS POTENTIAL STORM
SYSTEM IS THE VERY STRONG...COLD TROUGH JUST COMING OFF OF ASIA
ATTM. THE NRN PIECE OF THIS IS STILL FORECAST TO MOVE OVER A WEST
COAST RIDGE AND DIVE SE INTO THE CENTRAL ROCKIES OR INTER-MOUNTAIN
WEST CHRISTMAS EVE INTO CHRISTMAS DAY. MODELS HAVE BEEN VERY GOOD
WITH THE BIG PICTURE BUT ARE STRUGGLING WITH THE DETAILS WHICH HAVE
A SIGNIFICANT IMPACT ON TIMING AND DEPICTION OF HOW BIG THIS STORM
MAY BE FOR THE AREA. THE LAST TWO NIGHTS THEY HAVE ALL INDICATED A
PRETTY DECENT CHANCE OF NOT ONLY SNOW IN THE MOUNTAINS BUT EAST OF
THE DIVIDE ACROSS CENTRAL WYO SOMETIME BETWEEN CHRISTMAS EVE AND
INTO CHRISTMAS DAY. ECMWF CHANGED AGAIN IN ITS 00Z DEPICTION WITH A
FURTHER EAST DEVELOPMENT OF THE UPPER LOW. THE 12Z RUN WAS DEPICTING
A MAJOR SNOWSTORM EAST OF THE DIVIDE FOR THE END OF THIS PERIOD
THROUGH CHRISTMAS. THE GFS IS CHANGING EACH RUN ALSO WITH AN OPEN
WAVE PASSING THROUGH WITH THE NE HALF ON THE CYCLONIC SIDE OF THE
JET. ONE THING ALL THE RUNS CONTINUE TO SHOW IS A STRONG...COLD
CANADIAN UPPER HIGH DROPPING SOUTH IN THE 1040-1045MB RANGE
PROVIDING STRONG UPSLOPE EAST OF THE DIVIDE...AND PLENTY OF COLD
AIR. AS LONG AS YOU HAVE THIS FAVORABLE SURFACE PATTERN YOU HAVE
MORE ERROR TO PLAY WITH IN TERMS OF WHETHER IT WILL PRECIPITATE OR
NOT. HOW MUCH SNOW IS STILL UNCERTAIN AS IS THE EXACT TIMING BUT
IT DOES LOOK LIKE THERE WILL BE A DECENT SHOT OF SNOW MOVING INTO
THE WEST AND NORTH AGAIN CHRISTMAS EVE AND THEN POTENTIALLY SHIFT
FURTHER SOUTH RIGHT AT THE END OF THIS PERIOD AND THROUGH
CHRISTMAS DAY. STAY TUNED FOR MORE DETAILS AGAIN TONIGHT.

&&

.AVIATION.../18Z ISSUANCE/

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

VFR CONDITIONS WILL OCCUR AT MOST TERMINAL SITES TODAY AND
TONIGHT...EXCEPT FOR PERHAPS KRIW WHICH WILL BE ONE OF THE LAST
SPOTS TO CLEAR IFR TO LIFR AND IMPROVE TO MVFR EARLY THIS AFTERNOON.
CONDITIONS WILL OCCUR AT KRIW AIRPORT THROUGH MUCH OF THE DAY AS FOG
AND VERY LOW CEILINGS OCCUR. EVEN WITH IMPROVED CONDITION AT KRIW
THIS AFTERNOON...CIGS ARE LIKELY TO LOWER BY 03Z THURS ALONG WITH
MORE FOG POSSIBLE AT THE AIRPORT. KLND AIRPORT MAY ALSO SEE FOG AND
LOW CLOUDS AND MVFR CONDITIONS THROUGH 10Z THURSDAY.

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

LIGHT SNOW WILL GRADUALLY END BY 20Z ACROSS THE NORTHWEST MOUNTAINS.
HOWEVER...EXPECT MOUNTAINS TO BE FREQUENTLY OBSCURED THROUGH 00Z
THURSDAY. AT KJAC CIGS WILL BALANCE JUST ON EITHER SIDE OF VFR WITH
OCCASIONAL MVFR CEILINGS CONTINUING UNTIL THROUGH THE EVENING...THEN
SKIES WILL CLEAR AND FOG/STRATUS WITH MVFR CONDITIONS WILL PREVAIL
AFTER 06Z...THROUGH AT LEAST 14Z. OTHERWISE...THE SOUTHWEST AND
SOUTH WILL SEE VFR CONDITIONS PREVAILING WITH VCFG AROUND THROUGH
14Z THURSDAY.
&&

.FIRE WEATHER...

WINTER LIKE CONDITIONS CONTINUE ACROSS THE FORECAST AREA...KEEPING
TEMPS ON THE LOWER SIDE...AND RH VALUES ON THE HIGHER SIDE...WITH
FIRE DANGER OBVIOUSLY REMAINING LOW FROM NOW THROUGH MOST OF THE
WEEK. AGAIN...DUE TO THE COLDER NEAR SURFACE TEMPERATURES AND WEAK
WINDS....POOR MIXING AND SMOKE DISPERSION CAN ALSO BE EXPECTED TO
CONTINUE THROUGH MOST OF THE WEEK.
&&

.RIW WATCHES/WARNINGS/ADVISORIES...
DENSE FOG ADVISORY UNTIL 9 AM MST THIS MORNING WYZ010-017-018.

&&

$$

SHORT TERM...HATTINGS
LONG TERM...PS
AVIATION...JB
FIRE WEATHER...HATTINGS












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